The field of the invention is signal enhancement of photo detector outputs as seen, for example, in aerosol particle detection and measurement systems. In particular, the present invention is related to enhancement of electrical signals for the detection of light detected by a photodiode and more specifically by light detected by a photodiode as the result of light scattering from a concentration of aerosol particles.
Examples of systems in the background art that use photodetectors include, but are not limited to, optical character recognition systems, communication systems medical imaging sensors, laser range finders, radiation detectors, smoke detectors, position sensors and proximity sensors. In all of these background art systems, a photodetector is used to measure light or radiation in terms of an electrical signal that is processed in various ways to produce a useful information output. In a particular example from the background art, a beam of collimated light, which may or may not be coherent, is directed through a transparent cell in which particles suspended in fluid mixtures are made to pass. Photodetectors are then used to detect the relative amount of light that is scattered or blocked by the particles. The signals generated by the photodetector contain information about the concentration of particles, size of particles, and/or presence of particles.
The type of photodetector used depends on the sensitivity requirements of the device. A photo-multiplier tube is the most sensitive (and costly) method that is currently available. A photo-multiplier can detect the presence of a single photon with nanosecond resolution. However, photo-multiplier tubes are very costly to manufacture and are easily damaged. Additionally they have very high voltage requirements and therefore tend to be used in laboratories rather than in commercial applications.
One alternative to using a photo-multiplier tube is to use a photodiode and a transimpedance amplifier. In contrast to a photo-multiplier, photodiodes are inexpensive, rugged, small, and operate at low voltages.
Another background art device that is used to measure aerosol particle size and concentration is called a light scattering photometer or nephelometer. Applications that require particularly sensitive measurements require photo-multiplier-based photometers.
When the sensitivity requirements of the application do not justify the use of a photo-multiplier tube, a photodiode-based device is preferred due to the reduced cost. However, background art photodiodes are not as sensitive as photo-multiplier tubes and are prone to noise problems associated with electrical amplification.
The sensitivity of a photodiode device is in part a function of the gain of a transimpedance amplifier associated with the photodiode device. The amplified signals contain useful information pertaining to the amount of light reaching the photo detector. However, due to the inherent properties of the photo detector and amplifier circuits, the amplified signal also contains additional factors such as offset voltage potential, noise generated by ambient light and electromagnetic interference. These additional factors have the effect of limiting the possible gain of the amplifier stages before reaching saturation. Therefore, there is a need in the art for a low-cost photodiode-based detector with improved gain and sensitivity.